Fluorescence of adenine, guanine, cytosine, and uracil at room temperature in neutral aqueous solution has been detected by means of a digital signal accumulation technique. Corrected emission and excitation spectra are presented and compared with low-temperature data. The quantum yields are, respectively, 2.6 x 10(-4), 3.0 x 10(-4), 0.8 x 10(-4), and 0.5 x 10(-4) when the bases are excited at their low-energy absorption maxima.

Nitrate solutions have been photolyzed in the first weak band at 300 mp using the filtered output of a 450-W high-pressure mercury lamp. In neutral solution, nitrite formation is initially nonlinear with time, falling off to a low constant rate. This behavior has been investigated as a function of nitrate concentration, nitrite buildup, and addition of nitrite prior to photolysis, The results are quantitatively consistent with process A NO3--% NO2-+ 0 0 + Nos--% 0 2 + Not-and allow evaluation of kz/kl as -2 X lo4; 9 (NOP-)A 'v 4 X independent of intensity and nitrate concentration. Addition of radical scavengers (As(III), ethanol, H20z, and benzene) prior to photolysis causes drastic increase in rate of nitrite formation. Consideration of reaction stoichiometrics and mechanisms leads to the conclusion that the additives are scavenging OH radicals. The scavenging kinetics are described by the Jortner-Ottolenghi-Stein (JOS) equation, from which the scavenging rate constant ratios HzOz:EtOH:As(III) are derived and found to be the same as the OH radical rate constants. Accordingly, it is suggested that a second primary process, B, takes place.Nos-pH has a pronounced effect on the photolysis. Nitrite formation becomes linear with time and increases severalfold. Two regions of rapid change may be distinguished: J (1) pH 8-10; (2) pH 11-13. The behavior a t pH 8-10 may be accounted for by changes in process A. From pH 11 to 13, the change may be correlated with ionization of OH radical (pK = 11.9) formed in process B. At the plateau pH 10.3, a dependence of @(NO2-) on nitrate concentration is found, which can be described by a JOS treatment. IntroductionIn the ultraviolet region, aqueous solutions of nitrate show two distinct absorption bands, a weak one centered at -300 mp (e 7.1 at 304 mp @ I 25", f = 1.4 X and an intense band in the far-ultraviolet region

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